THE STRUCTURAL CLASSIFICATION 571 



conclusion was reached in part by personal examination of samples and 

 in part by talking with the drillers of the wells. 



The question then arose whether hydrostatic or hydrologic conditions 

 may not have been responsible for the position of the accumulations. To 

 a certain extent this seemed to be true, since many of the oil pools in the 

 Berea sand are bounded on their down-dip sides by pools of salt water; 

 and following the hypothesis further, it was found that still farther down 

 the dip other oil and gas pools existed, which in turn were usually 

 bounded by salt-water pools on their down-dip sides. So far as could be 

 learned, the position of the water pools, rather than the oil or gas pools, 

 was determined by a retaining terrace or barrier of close-grained sand. 

 The best known examples of this condition are in the Pan-Handle of 

 West Virginia and in Oklahoma and Kansas, where pool after pool has 

 been discovered by the oil men, the last to be discovered being farthest 

 down dip. All, however, appear to be dependent predominantly on 

 structure. 



A similar condition of affairs was found in the development of the 

 Clinton sand fields of central Ohio. The Brem.en pool is situated down 

 dip from the main gas belt, and still farther do^vn dip is the Junction City 

 pool. Similar relations prevail throughout the Clinton fields. Between 

 all of the pools barren areas exist for short distances. While it is true 

 that in some of these barren areas the Clinton sand was found to be 

 locally hard, thin, or close, it is equally true that in the great majority 

 of cases it maintains its wonderfully uniform character without regard 

 to whether it holds oil or is dry. 



One other important factor remained to be considered, namely, stimc- 

 ture. When this was worked, out in detail for any particular field, the con- 

 clusion was reached in nearly every case that the peculiar structures asso- 

 ciated with the oil pools are too multifold in their correspondence to be 

 caused by mere chance agreements. When considered in a hroad light, 

 the common feature of most oil pools on monoclinal dips seems to he 

 their occurrence at points of interruption in the general rate of dip. 



Types of interruption. — On a monoclinal dip there seem to be two 

 main types of interruptions : ( 1 ) Those due to longitudinal warping, 

 parallel with the direction of strike of the sand, and (2) those due to 

 lateral warping, parallel with the direction of dip. The last-mentioned 

 type produces structural "noses," as illustrated in figure 8. The first 

 type of interruption produces a "ravine" or "notch" in the monoclinal 

 slope, such warpings being common in the Ohio fields. The ravine-like 

 structure, being a conspicuous type of abnormality in monoclinal dip, 

 seems to be very favorable for oil occurrence where the sands are dry. 



XLIV — Bull. Geol, Soc, Am., Vol. 28, 1916 



